This invention relates to an IC card comprising a solar battery, a keyboard, a display unit, etc. Specifically, the the present invention relates to the configuration of a switching circuit between the external power supply terminal and the solar battery of the card, and relates to protection of the solar battery vulnerable to static electricity or the like.
An example of an IC card employing a solar battery, a keyboard and a display unit is disclosed in Japanese Patent Application Laid-open No. 1986/5389. FIG. 1A and FIG. 1B are front and rear perspective views showing such an IC card. This card comprises a solar battery or solar cell and a secondary battery, and thereby enable data input through the keyboard, processing of such data by means of the CPU and display of such data by means of the display unit. The keyboard may include a key for transaction (TR), a key for reference or inquiry (RE) and a key for start (ST).
Moreover, the IC card has an external terminal, so that it can be connected, for example, to a bank terminal, such as a teller machine, and thus can be used as an electronic pass book.
When the IC card connected to an external equipment such as a bank terminal, is used in an on-line mode the power for the IC, the display unit, etc. of the card is supplied from the external equipment through the external power supply terminal. When the card is used in an off-line mode, in which the card is not connected to an external equipment, the power for the IC, the display unit, etc. of the card is supplied from the internal solar battery of the card.
Therefore, the IC card normally employs a configuration in which the external power supply terminal is directly connected to the IC and the display unit of the card, and the solar battery is also directly connected to the IC and the display unit of the card and hence to the external power supply terminal.
However, the solar batteries, especially amorphous solar batteries which are widely being used recently, are vulnerable to reverse voltages, and particularly to static voltages. Therefore, the reverse withstand voltage against static electricity is at most of the order of a few tens of volts, which is extremely small compared with the voltage of a few hundred volts of ICs formed of monocrystalline silicon and having provision of protection against static electricity. In addition, it must also be considered that such cards may be used in an environment where static electricity is easy to occur. For example the cards are used as electronic pass books and inserted in a clothing pocket.